ath10k: Enable MU MIMO txbf support for QCA99X0

This patch enables MU-MIMO transmit beamforming support
for QCA99X0 chipsets.

Signed-off-by: Vivek Natarajan <nataraja@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>